A geosynchronous satellite is a satellite that orbits the Earth and follows the direction of the Earth's rotation. One revolution of a geosynchronous satellite around the Earth takes about 24 hours, which is the same amount of time it takes for the Earth to rotate once about its axis. These types of satellites are considered geosynchronous because they appear stationary when viewed from a particular location on the Earth, and are commonly used as communication satellites.
The geosynchronous satellites have assigned orbits above the Earth's equator so they do not collide with one another or interfere with each other's communications. Geosynchronous satellites orbit at a radius of about 42,164 kilometers from the center of the Earth. Satellites at this radius make one revolution around the Earth in about 24 hours (a sidereal day) due to the gravitational force of the Earth. The orbit for a satellite may be affected by perturbations, such as gravitational forces from the Sun and Moon, the non-circular shape of the Earth, solar radiation pressure, etc. To negate the perturbations and keep a satellite in its assigned orbit, propulsion systems on the satellite perform active maneuvers that are referred to as “stationkeeping” maneuvers. When observed from a location on the Earth, the position of a satellite is maintained within an assigned orbital station or “box” which has predetermined dimensions. Stationkeeping involves control of the satellite's longitude, the eccentricity of its orbit, and the inclination of its orbital plane from the Earth's equatorial plane.
An example of stationkeeping is described in U.S. Pat. No. 6,015,116, which issued on Jan. 18, 2000. The propulsion system described in '116 uses four thrusters that are diagonally arranged on the back (zenith) side of the satellite. One pair of the thrusters have thrust lines directed through the center of mass of the satellite, while the other pair have thruster forces that are spaced by momentum arms from the center of mass. Another example of stationkeeping is described in U.S. Pat. No. 7,918,420, which issued on Apr. 5, 2011. Both of the patents are incorporated by reference as if fully included herein.
It is desirable to identify new and improved stationkeeping maneuvers that are effective yet fuel efficient.